This invention relates generally to scintillator screens for converting radiation to visible light and, more particularly, to phosphor input layers or X-ray image intensifier tubes.
As part of a diagnostic imaging system, an image intensifier tube is used to convert the incident X-ray information to a visible light image of increased brightness. Such an image intensifier tube typically includes at its input end, a faceplate, a phosphor layer or screen for absorbing the incident X-ray photons and for emitting light photons, and a photocathode or photo-emitting layer which absorbs the light photons from the phosphor and emits photoelectrons. The tube acts to accelerate and focus these photoelectrons to form an intensified or brightened image on the output screen of the tube. The brightened image is then processed with conventional means to obtain display on a monitoring screen.
One of the factors which affects the quality of the final image is the thickness of the fluorescent screen. Generally, it is desirable to have the phosphor coating thick enough to absorb all of the X-rays and to thereby avoid excessive electron penetration or "crutch through" of the phosphor that causes excessive noise; however, as the phosphor layer is made thicker, both the resolution and the contrast are decreased because of a phenomena known as "lateral spreading." So, a trade off must be made to have the X-ray screen thick enough to absorb all of the X-ray quanta but, on the other hand, thin enough in order to give a well-resolved image.
In addition to the thickness consideration, the lateral spreading phenomena may also be affected by changes in the processes by which the screens are made. And, therefore, changes in the actual structure of the scintillator screens. In the development of this technology, it has been found that vapor deposition techniques result in higher packing density of phosphor material than could be realized by the conventional tarter preparation procedures. A preferred material for this vapor deposition technique has been cesium iodide or, more specifically, CsI:Na. Moreover, the resolution of a vapor-deposited screen can be improved by suppression of lateral scattering of generated light. Recent development in this field has brought about the use of structured substrates, such as selectively etched metal sheets or wire gauzes or the like for producing regular crack patterns perpendicular to the surface so as to act as light barriers. Screens having the largest number of light barriers of this type are expected to produce the best resolution. The next step in this process has been to anneal the deposit layer to eliminate the electron traps which may exist and which cut down on the light output of the screen. Typical annealing cycles for conventional screens are from one-half to several hours at a temperature range of 200.degree. C. In addition to the desirable elimination of electron traps, this process tends to cause a re-crystallization of the individual phosphor fibers. Because of their close proximity to one another, the fibers have a tendency to fuse together, and, when this occurs, tube resolution suffers. Typically, a screen is formed by alternately depositing layers and annealing layers to form multiple and serial depositions which finally extend to the desired thickness.
In the normal process of vapor deposition, the source is located directly under the substrate on which it is deposited. As the phosphor vapor rises to the substrate, it first contacts the central portion thereof and then flows to the peripheral portions thereof. Accordingly, the resulting deposit layer of phosphor tends to be thicker in the central region than at the edges of the screen. Such a screen with thinner edges is undesirable because it tends to enhance the inherent loss of image brightness which exists at the edge of a screen due to the nature of the tube electron-optic designs. Thus, instead of having thinner edges on the screens, it would be preferable to have thicker edges to offset this phenomena.
In view of the problems and deficiencies discussed hereinabove, it is therefore an object of the present invention to provide an improved scintillator screen and method of making such a screen.
Another object of the present invention is a provision for fabricating a scintillator screen having improved quantum-detection efficiency (QDE) characteristics.
Still another object of the present invention is the provision for fabricating a scintillator screen which is substantially comprised of a plurality of columnar elements extending substantially perpendicularly from a substrate.
Still another object of the present invention is the provision for fabricating a scintillator screen with minimal re-crystallization caused by annealing.
Yet another object of the present invention is the provision for a scintillator screen having a single layer of phosphor material deposited at a sufficient thickness to provide relatively high quantum-detection efficiency characteristics.
Still another object of the present invention is the provision for a scintillator screen structure having a greater thickness at its edges than in its center.
These objects and other features and advantages become more readily apparent on reference to the following description when taken in conjunction with the appended drawings.